Method for the continuous production of propylene glycol

ABSTRACT

Process for the continuous preparation of propylene glycols, which comprises the steps (i) to (ii):
         (i) reacting propylene with hydrogen peroxide to form propylene oxide and give propylene glycols as by-products,   (ii) reacting the propylene oxide obtained in step (i) with water to give propylene glycols,   (iii) separating off the propylene glycols obtained in steps (i) and (ii).

The present invention relates to a process for the continuouspreparation of propylene glycols by reaction of propylene with hydrogenperoxide to give propylene oxide with simultaneous or subsequent glycolformation and joint work-up. The glycols are separated from the productmixture by distillation, with propylene glycol and dipropylene glycolbeing separated off in a dividing wall column having two side offtakesand tripropylene glycol being separated off in a column which isthermally coupled with the dividing wall column. The invention alsorelates to an apparatus for carrying out the process.

Propylene glycol and polypropylene glycols are important intermediates,for example, for producing polyester and epoxy resins and alsopolyurethane forms. Their industrial preparation has hitherto mostlybeen carried out using a 3-stage process.

In this process, propylene oxide is reacted with a stoichiometric excessof water at from 180 to 220° C. and a pressure of from 15 to 25 bar in afirst stage. Tube reactors connected in series are generally used forthis reaction. The propylene glycols obtained are then present in theform of a mixture comprising propylene glycol, also known asmonopropylene glycol or 1,2-propylene glycol, dipropylene glycol andtripropylene glycol in a weight ratio of about 100:10:1. This mixture istransferred, in a second stage, to a dewatering apparatus in which theexcess water is separated off in a multistage evaporation and isreturned to the reaction. Finally, in a third stage, the propyleneglycols are distilled to give pure products. According to the prior art,this is carried out using three columns which are connected in seriesand each have a side offtake. In the first column, the propylene glycolis obtained from the side offtake and the bottoms are passed to thesecond column from whose side offtake the dipropylene glycol isobtained. The mixture from the bottom of this second column is passed tothe third column from whose side offtake the tripropylene glycol canfinally be obtained (Ullmann's Encyclopadie der technischen Chemie, 4thEdition, Volume 19, pages 425–432).

The process described requires a high outlay in terms of apparatus andenergy, particularly in the process step in which the three propyleneglycols mentioned are separated by distillation, since three columns areused. Furthermore, it requires the use of pure propylene oxide asstarting material.

It is an object of the present invention to provide an improved processfor preparing the propylene glycols. In particular, the process of thepresent invention should make it possible to use propylene oxide of lowpurity as starting material and allow the separation of the propyleneglycols mentioned to be carried out with a reduced outlay in terms ofapparatus and energy.

We have found that this object is achieved by a process in which thepreparation of propylene oxide in which some propylene glycols areformed as by-products is coupled with the reaction of the propyleneoxide with water. The propylene oxide is preferably prepared by reactionof propylene with peroxides, for example, hydrogen peroxide. Thepropylene glycols are separated off by distillation in a dividing wallcolumn having two liquid side offtakes and a column which is thermallycoupled therewith.

The present invention accordingly provides a process for the continuouspreparation of propylene glycols, which comprises at least the steps (i)to (iii):

-   -   (i) reacting propylene with hydrogen peroxide to form propylene        oxide and propylene glycols,    -   (ii) reacting the propylene oxide obtained in step (i) with        water to give propylene glycols,    -   (iii) separating off the propylene glycols obtained in steps (i)        and (ii).

In a preferred embodiment, the separation in step (iii) is carried outby distillation in a dividing wall column having two side offtakes and acolumn which is thermally coupled therewith, with propylene glycol beingobtained from the upper side offtake of the dividing wall column,dipropylene glycol being obtained from the lower side offtake andtripropylene glycol being obtained from the column which is thermallycoupled therewith.

In the novel process, it is thus no longer necessary to use purepropylene oxide in the reaction with water, but instead it is possibleto use the crude product, which already contains amounts of thepropylene glycols, obtained in the synthesis of propylene oxide frompropylene and hydrogen peroxide. The use of the dividing wall column incombination with the thermally coupled column can, in addition, enableconsiderable savings to be achieved in terms of apparatus and energyconsumption compared to the prior art, since only two columns instead ofthree are necessary for separating off the glycols. The continuousprocess thus offers advantages compared to the process of the prior art.

The preparation of propylene oxide by reaction of propylene withhydrogen peroxide in step (i) is known and can be carried out by knownmethods. Both single-stage and multistage processes can be used. Owingto their economic advantages multistage processes are preferred.

For example, the multistage process described in WO 00/07965 providesfor the reaction of propylene with hydrogen peroxide to form propyleneoxide to comprise at least the steps (α) to (γ):

-   -   (α) reacting propylene with hydrogen peroxide to give a mixture        comprising propylene oxide and unreacted hydrogen peroxide,    -   (β) separating the unreacted hydrogen peroxide from the mixture        resulting from step (α),    -   (γ) reacting the hydrogen peroxide which has been separated off        in stage (β) with propylene.

Accordingly, in this reaction sequence, the reaction of propylene withhydrogen peroxide takes place in at least two steps (α) and (γ), withthe hydrogen peroxide separated off in step (β) being used in thereaction again.

In this sequence, said reaction is preferably carried out over aheterogeneous catalyst, optionally using a solvent such as methanol.

The reactions in steps (α) and (γ) preferably take place in two separatereactors. Fixed-bed reactors are usually employed for this purpose, withthe reaction of step (α) being carried out in an isothermal reactor andthe reaction of step (γ) being carried out in an adiabatic reactor.

The hydrogen peroxide conversion in step (α) is generally from about 85%to 90% and that in step (γ) it is about 95% based on step (β). Overall,a hydrogen peroxide conversion of about 99% over both stages can beachieved at a propylene oxide selectivity of about 94–95%.

The mixture obtained in the reaction generally contains from about 6 to10% by weight of propylene oxide. Further components present are, forexample, methanol as solvent, water, unreacted propylene as organiccompound and hydrogen peroxide and also acetaldehyde and methyl formate.

The water present in the product mixture originates from the reaction ofthe hydrogen peroxide with propylene and also decomposition of thehydrogen peroxide. Furthermore, an aqueous hydrogen peroxide solution ispreferably used in steps (α) and (γ). Under the prevailing conditions,water may react with propylene oxide to form propylene glycol. Theglycol formed in this way as by-product can in turn react further withadditional propylene oxide to give dipropylene glycol and tripropyleneglycol. Thus, monopropylene glycol, dipropylene glycol and tripropyleneglycol are present in addition to the propylene oxide. These glycols canreadily be separated by distillation from the product mixture containingthe propylene oxide since they have relatively high boiling points. Whenthe distillation is carried out in a column, they are obtained in thebottoms while propylene oxide together with further volatile componentsis distilled off via the top of the column. This propylene oxide has apurity of from about 95 to 99% and is referred to as crude propyleneoxide.

The 1- and 2-methoxypropanols obtained as further by-products bysubsequent reaction of propylene oxide with methanol under the reactionconditions are also obtained in the bottoms. Further by-products presentin the bottoms are 2-hydroperoxy-1-propanol and 1-hydroperoxy-2-propanolwhich are formed by addition of hydrogen peroxide on to propylene oxide.

These hydroperoxy alcohols are preferably reduced, likewise formingpropylene glycol. For example, the methods described in DE 101 05 527.7can be used for the reduction. A preferred reduction method is catalytichydrogenation.

In one embodiment of the process of the present invention, the propyleneglycol obtained as by-product in step (i) is accordingly formed byreduction of 2-hydroperoxy-1-propanol and 1-hydroperoxy-2-proanol.

There is therefore a second possible way of propylene glycol beingformed as by-product in addition to the reaction of propylene oxide withwater.

For the purposes of further reaction, it is extremely advantageous forthe crude propylene oxide to be able to be used in the process of thepresent invention without further purification.

The crude propylene oxide is then reacted with water to give a mixtureof propylene glycols. The reaction can advantageously be carried outusing the process described in the prior art and the reaction conditionsand reactors indicated therein. However, all possible processes for thecatalytic addition of water on to propylene oxide can also be employed,for example, the method described in WO 99/31034.

The crude propylene oxide is preferably reacted with a stoichiometricexcess of water at from 180 to 220° C. and a pressure in the range from15 to 25 bar, for instance in a tube reactor or tube reactors connectedin series. The propylene glycols formed are then present in the form ofa mixture consisting essentially of propylene glycol, dipropyleneglycol, tripropylene glycol and polypropylene glycols.

In one embodiment of the process of the present invention, propyleneoxide is therefore reacted with water at from 180 to 220° C. and apressure of from 15 to 25 bar in step (ii).

The residence times in the reactor or reactors depend essentially on thedesired conversions. They are generally less than 5 hours, preferablyless than 3 hours, more preferably less than 1 hour and particularlypreferably about half an hour.

The mixture obtained in step (ii) is preferably transferred to adewatering apparatus in which the excess water is separated off in amultistage evaporation. This water can be returned to the reaction withthe propylene oxide. As apparatuses for dewatering, it is possible touse conventional distillation columns in which the water is distilledoff via the top as a low-boiling fraction. The glycol mixture can beisolated at the bottom or via the side offtake of such apparatuses.

In a preferred embodiment of the process of the present invention, thewater is accordingly removed from the mixture obtained in step (ii)before the propylene glycols are separated off in step (iii).

The mixture obtained in the dewatering step preferably has a residualwater content of less than 5% by weight before it is transferred to theapparatus in which the propylene glycols are to be separated off.

The glycols obtained in step (i) and step (ii) are then separated off instep (iii). Here, the glycol mixture which has been obtained andseparated off as secondary yield in step (i), as described above, iscombined with the glycol mixture obtained from the dewatering step andthe two are distilled together to separate off the individual glycols.

Before the glycol mixtures from step (i) and step (ii) are combined, themethoxypropanols and the solvent, namely, methanol, present in themixture from step (i) are preferably separated off.

According to the present invention, a feed stream comprising the glycolmixture is then fed continuously into a dividing wall column. Accordingto the present invention, use is made of a dividing wall column havingtwo side offtakes which are located one above the other and a columnwhich is thermally coupled therewith and into which a stream from thelower region of the dividing wall column is fed. The thermally coupledcolumn is a conventional rectification column which is operated withouta vaporizer. The heat required for operation of this column isintroduced via the vapor stream.

Dividing wall columns are known from the prior art. They represent afurther development of distillation columns which have only a sideofftake but no dividing wall. The use of the last-named conventionaltype of column is, however, restricted because the products taken off atthe side offtakes are never completely pure. In the case of productstaken off at side offtakes in the enrichment section of the column,which are usually taken off in liquid form, the side product stillcontains proportions of low-boiling components which should be separatedoff via the top. In the case of products taken off at side offtakes inthe stripping section of the column, which are usually taken off ingaseous form, the side product still contains proportions of highboilers. The use of conventional side offtake columns is thereforerestricted to cases in which contaminated side products are permissible.

When a dividing wall is installed in such a column, the separationaction can be improved and it becomes possible for side products to betaken off in pure form. A dividing wall is installed in the middleregion above and below the feed point and the side offtakes. This can befixed in place by welding or can be merely pushed into place. It sealsoff the offtake part from the inflow part and prevents crossmixing ofliquid and vapor streams over the entire column cross section in thispart of the column. This reduces the total number of distillationcolumns required in the fractionation of multicomponent mixtures whosecomponents have similar boiling points.

This type of column has been used, for example, for the separation of aninitial mixture of the components methane, ethane, propane and butane(U.S. Pat. No. 2,471,134), for the separation of a mixture of benzene,toluene and xylene (U.S. Pat. No. 4,230,533), for the separation of amixture of n-hexane, n-heptane and n-octane (EP 0 122 367), for theseparation of azeotropically boiling mixtures (EP 0 133 510) and forcarrying out chemical reactions with simultaneous distillation of theproducts (EP 0 126 288).

Such a dividing wall column, as shown schematically in the FIGURE,preferably has from 15 to 60, more preferably from 20 to 40, theoreticalplates. The process of the invention can be carried out particularlyadvantageously using such a design.

The upper combined region 1 of the inflow and offtake part of the columnpreferably has from 5 to 50%, more preferably from 15 to 30%, theenrichment section 2 of the inflow part preferably has from 5 to 50%,more preferably from 15 to 30%, the stripping section 4 of the inflowpart preferably has from 5 to 50%, more preferably from 15 to 30%, thestripping section 3 of the offtake part preferably has from 5 to 50%,more preferably from 15 to 30%, the enrichment section 5 of the offtakepart preferably has from 5 to 50%, more preferably from 15 to 30%, thelower combined region 6 of the inflow and offtake part of the columnpreferably has from 5 to 50%, more preferably from 15 to 30%, and theregion of thermal coupling 7 of the two side offtakes preferably hasfrom 5 to 50%, more preferably from 15 to 30%, in each case of the totalnumber of theoretical plates in the column.

The sum of the number of theoretical plates in the regions 2 and 4 inthe inflow part is preferably from 80 to 110%, more preferably from 90to 100%, of the sum of the number of theoretical plates in the regions3, 5 and 7 in the offtake part.

It is likewise advantageous for the feed point Z and the side offtakesfrom which the glycols PG and DPG are taken to be arranged at differentheights in the column relative to the position of the theoreticalplates. The feed point Z is preferably located at a position which isfrom 1 to 8, more preferably from 3 to 5, theoretical plates above orbelow the side offtakes.

The dividing wall column used in the process of the present invention ispreferably configured as a packed column containing random packing orordered packing or as a tray column. For example, it is possible to usesheet metal or mesh packing having a specific surface area of from 100to 1000 m²/m³, preferably from about 250 to 750 m² m³, as orderedpacking. Such packing provides a high separation efficiency combinedwith a low pressure drop per theoretical plate.

In the abovementioned configuration of the column, the region of thecolumn divided by the dividing wall 8, which consists of the enrichmentsection 2 of the inflow part, the stripping section 3 of the offtakepart, the stripping section 4 of the inflow part and the enrichmentsection 5 of the offtake part or parts thereof is/are provided withordered packing or random packing. The dividing wall can be thermallyinsulated in these regions.

The product stream is advantageously metered by means of a pump or via astatic inflow head of at least 1 m into the inflow part. This inflow ispreferably regulated via a cascade regulation in combination with theregulation of the liquid level in the inflow part. The regulation is setso that the amount of liquid introduced into the enrichment sectioncannot drop below 30% of the normal value. It has been found that such aprocedure is important to even out troublesome fluctuations in theamount or concentration of the feed.

It is likewise important for the division of the liquid flowing downfrom the stripping section of the offtake part of the column between theside offtake and the enrichment section of the offtake part to be set bymeans of a regulation device so that the amount of liquid going to theenrichment section cannot drop below 30% of the normal value.

Adherence to these prerequisites has to be ensured by means ofappropriate regulation methods.

Regulation mechanisms for the operation of dividing wall columns havebeen described, for example, in Chem. Eng. Technol. 10 (1987) 92–98,Chem.-Ing.-Technol. 61 (1989) No. 1, 16–25, Gas Separation andPurification 4 (1990) 109–114, Process Engineering 2 (1993) 33–34, TransIChemE 72 (1994) Part A 639–644, Chemical Engineering 7 (1997) 72–76).The regulation mechanisms described in this prior art can also beemployed for or applied to the process of the present invention.

The regulation principle described below has been found to beparticularly useful for the continuously operated isolation of theglycols. It is readily able to cope with fluctuations in loading. Thedistillate is thus preferably taken off under temperature control.

A temperature regulation device which utilizes the downflow quantity,the reflux ratio or preferably the quantity of runback as regulatingparameter is provided in the upper section 1 of the column. Themeasurement point for the temperature regulation is preferably locatedfrom 3 to 8, more preferably from 4 to 6, theoretical plates below theupper end of the column.

Appropriate setting of the temperature then results in the liquidflowing down from the upper section 1 of the column being divided at theupper end of the dividing wall 8 so that the ratio of the liquid flowingto the inflow part to that flowing to the offtake part is preferablyfrom 0.1 to 1.0, more preferably from 0.3 to 0.6.

In this method, the down flowing liquid is preferably collected in areceiver which is located in or outside the column and from which theliquid is then fed continuously into the column. This receiver can thustake on the task of a pump reservoir or provide a sufficiently highstatic head of liquid which makes it possible for the liquid to bepassed on further in a regulated manner by means of regulating devices,for example, valves. When packed columns are used, the liquid is firstlycollected in collectors and from there conveyed to an internal orexternal receiver.

The vapor stream at the lower end of the dividing wall 8 is set byselection and/or dimensioning of the separation internals and/orinstallation of pressure-reducing devices, for example, orifice plates,so that the ratio of the vapor stream in the inflow part to that in theofftake part is preferably from 0.8 to 1.2, preferably from 0.9 to 1.1.

In the abovementioned regulation principle, a temperature regulationdevice which utilizes the quantity taken off at the bottom as regulatingparameter is provided in the lower combined section 6 of the column. Thebottom product can thus be taken off under temperature control. Themeasurement point for the temperature regulation device is preferablylocated from 3 to 6, more preferably from 4 to 6, theoretical platesabove the lower end of the column.

In addition, the level regulation in the bottom of the column can beprovided as regulating parameter for the abovementioned regulation ofthe quantity taken off at the lower side offtake and a temperatureregulation in the divided column region can be provided for the quantitytaken off at the upper side offtake.

The differential pressure over the column can also be used as regulatingparameter for the heating power. The distillation is advantageouslycarried out at a pressure at the top of from 5 to 500 mbar, preferablyfrom 10 to 200 mbar. Accordingly, the heating power of the vaporizer atthe bottom of the column is selected to maintain this pressure range.

Under these pressure conditions, the distillation temperature is then inthe range from 50 to 200° C., preferably from 80 to 180° C., measured atthe side offtakes.

To be able to operate the dividing wall column in a trouble-free manner,the abovementioned regulation mechanisms are usually employed incombination.

The sum of the number of theoretical plates in the enrichment section 2and stripping section 4 in the inflow part is preferably from 80 to110%, more preferably from 90 to 100%, of the sum of the number oftheoretical plates in the enrichment section 3 and stripping section 5in the offtake part and the region of thermal coupling 7 of the sideofftakes.

The division of the liquid flowing down from the stripping section 3 ofthe offtake part of the column between the side offtake and theenrichment section 5 of the offtake part is set by means of a regulationdevice so that the quantity of liquid flowing into the enrichmentsection cannot drop below 30% of the normal value.

For example, the product stream in which the propylene glycols arepresent can be divided by means of the regulation principle described sothat the low boilers LS comprising, for example, water and solvent, canbe taken off via the top of the dividing wall column and be condensed inthe condenser K. The intermediate boilers are taken off via the upperside offtake in the form of propylene glycol PG and via the lower sideofftake in the form of dipropylene glycol DPG. The high boilers HScomprising the tripropylene glycol TPG can be obtained at the bottom ofthe column.

In the separation of multicomponent mixtures into low-boiling,intermediate-boiling and high-boiling fractions, there are usuallyspecifications in respect of the maximum permissible proportion of lowboilers and high boilers in the middle fraction. Here, individualcomponents which are critical to the separation problem, referred to askey components, or else the sum of a plurality of key components are/isspecified.

Adherence to the specification for the high boilers in theintermediate-boiling fraction is preferably regulated via the divisionratio of the liquid at the upper end of the dividing wall 8. Thedivision ratio is set so that the concentration of key components forthe high-boiling fraction in the liquid at the upper end of the dividingwall amounts to from 10 to 80% by weight, preferably from 30 to 50% byweight, of the value which is to be achieved in the stream taken off atthe side. The liquid division can then be set so that when theconcentration of key components of the high-boiling fraction is higher,more liquid is introduced into the inflow part, and when theconcentration of key components is lower, less liquid is introduced intothe inflow part.

Accordingly, the specification for the low boilers LS in theintermediate-boiling fraction is regulated by means of the heatingpower. Here, the heating power in the vaporizer V is set so that theconcentration of key components for the low-boiling fraction in theliquid at the lower end of the dividing wall 8 amounts to from 10 to 80%by weight, preferably from 30 to 50% by weight, of the value which is tobe achieved in the product taken off at the side. Thus, the heatingpower is set so that when the concentration of key components of thelow-boiling fraction is higher, the heating power is increased, and whenthe concentration of key components of the low-boiling fraction islower, the heating power is reduced.

The concentration of low and high boilers in the intermediate-boilingfraction can be determined by customary analytical methods. For example,infrared spectroscopy can be used for detection, with the compoundspresent in the reaction mixture being identified by means of theircharacteristic absorptions. These measurements can be carried outin-line directly in the column. However, preference is given to usinggas-chromatographic methods. In this case, sampling facilities are thenprovided at the upper and lower end of the dividing wall. Liquid orgaseous samples can then be taken off continuously or at intervals fromthe column and analyzed to determine their compositions. The appropriateregulation mechanisms can then be activated as a function of thecomposition.

An objective of the process of the present invention is to provide theintermediate boilers propylene glycol PG and dipropylene glycol DPG in apurity in each case of preferably greater than 95%, more preferablygreater than 99%.

A mixture comprising the tripropylene glycol TPG is then taken from thebottom of the dividing wall column and passed to a distillation columnwhich is thermally coupled with the dividing wall column.

According to the present invention, energy is introduced via thedividing wall column by means of the vaporizer V. Vapor and liquidexchange d and f occurs in the lower region of the dividing wall columnand thus of the column which is thermally coupled therewith.

In the thermally coupled column, the tripropylene glycol TPG can bedistilled off via the top of the column at a pressure of preferably from5 to 500 mbar, more preferably from 10 to 200 mbar, and a temperature ofpreferably from 100 to 200° C., more preferably from 120 to 180° C. Itis then condensed in the condenser K in which the stream of dipropyleneglycol DPG can be used as coolant. The tripropylene glycol is preferablyobtained in a purity of at least 95%.

Products having a boiling point higher than that of tripropylene glycolTPG which condense out are recirculated together with the bottoms fromthe thermally coupled column as liquid stream f to the dividing wallcolumn. There, they can finally be taken off as high boilers HS in thebottoms from the column and passed, for example, to incineration.Examples of such bottom products are polypropylene glycols.

The thermally coupled column in which the tripropylene glycol isseparated off can likewise be configured as a packed column containingrandom packing or ordered packing or as a tray column. For example, itis possible to use sheet metal or mesh packing having a specific surfacearea of from 100 to 1000 m²/m³, preferably from about 250 to 750 m²/m³,as ordered packing. Such packing provides a high separation efficiencycombined with a low pressure drop per theoretical plate.

The coupling of the continuous preparation of propylene oxide with thepreparation of the propylene glycols, which is likewise carried outcontinuously, helps make the continuous process of the present inventionextraordinarily advantageous for industrial use. For the purposes of thepresent invention, continuous means that the steps (i), (ii) and (iii)of the process of the present invention are carried out simultaneously.

A backward-integrated process starting from propylene is thus providedfor the continuous preparation of propylene glycol, dipropylene glycoland tripropylene glycol. Since the purification by distillation of thepropylene oxide and the distillation in three columns described in theprior art are dispensed with, a comparative saving in energy costs ofabout 30% can be achieved together with a reduced layout in terms ofapparatus.

The process of the present invention will be outlined once more with theaid of the FIGURE.

In zone A, hydrogen peroxide is reacted with propylene P to formpropylene oxide. This gives, after work-up, crude propylene oxide PO(1)and the propylene glycols G as secondary yield.

The crude propylene oxide PO(1) is then passed to the zone C where it isreacted with water to give propylene glycols. In zone D the water isremoved from these and can recirculated to the reaction with propyleneoxide PO(1) to form propylene glycols.

The stream obtained from zone D is combined with the secondary yield ofpropylene glycols G and fed into the dividing wall column via the feedpoint Z. Propylene glycol PG and dipropylene glycol DPG are separatedoff in the dividing wall column and triproylene glycol TPG is separatedoff in the column which is thermally coupled therewith.

It is also possible to branch off a substream from the product streamPO(1) and process it in zone B to give pure propylene oxide PO(2) whichcan, for example, have a purity of more than 99.9%.

“Chemical grade” propylene can be used for the process of the presentinvention. Such propylene contains propane and has a volume ratio ofpropylene to propane of from about 97:3 to 95:5.

The hydrogen peroxide used for the reaction is preferably in the form ofan aqueous hydrogen peroxide solution. It can be prepared, for example,by means of the anthraquinone process as described in “Ullmann'sEncyclopedia of Industrial Chemistry”, 5th Edition, Volume 13, pages 447to 456.

It is likewise conceivable to obtain hydrogen peroxide by convertingsulfuric acid into peroxodisulfuric acid by anodic oxidation withsimultaneous evolution of hydrogen at the cathode. Hydrolysis of theperoxodisulfuric acid then leads via peroxomonosulfuric acid to hydrogenperoxide and sulfuric acid, which is thus recovered.

It is of course also possible to prepare hydrogen peroxide from theelements.

As solvents for the preparation of propylene oxide, it is preferablypossible to use all solvents which completely or at least partlydissolve the starting materials used in the oxirane synthesis. Examplesof solvents which can be used are water; alcohols, preferably loweralcohols, more preferably alcohols having less than 6 carbon atoms, forexample, methanol, ethanol, propanols, butanols, pentanols, diols orpolyols, preferably those having less than 6 carbon atoms; ethers suchas diethyl ether, tetrahydrofuran, dioxane, 1,2-diethoxyethane,2-methoxyethanol; esters such as methyl acetate or butyrolactone; amidessuch as dimethylformamide, dimethylacetamide, N-methylpyrrolidone;ketones such as acetone; nitriles such as acetonitrile; sulfoxides suchas dimethyl sulfoxide; aliphatic, cycloaliphatic and aromatichydrocarbons, or mixtures of two or more of the abovementionedcompounds.

Preference is given to using alcohols. The use of methanol as solvent isparticularly preferred.

As catalysts for the preparation of propylene oxide, preference is givento using those which comprise a porous oxidic material, e.g. a zeolite.The catalysts used preferably comprise a titanium-, germanium-,tellurium-, vanadium-, chromium-, niobium- or zirconium-containingzeolite.

Specific mention may be made of titanium-, germanium-, tellurium-,vanadium-, chromium-, niobium- and zirconium-containing zeolites havinga pentasil zeolite structure, in particular the types which can beassigned X-ray-crystallographically to the ABW, ACO, AEI, AEL, AEN, AET,AFG, AFI, AFN, AFO, AFR, AFS, AFT, AFX, AFY, AHT, ANA, APC, APD, AST,ATN, ATO, ATS, ATT, ATV, AWO, AWW, BEA, BIK, BOG, BPH, BRE, CAN, CAS,CFI, CGF, CGS, CHA, CHI, CLO, CON, CZP, DAC, DDR, DFO, DFT, DOH, DON,EAB, EDI, EMT, EPI, ERI, ESV, EUO, FAU, FER, GIS, GME, GOO, HEU, IFR,ISV, ITE, JBW, KFI, LAU, LEV, LIO, LOS, LOV, LTA, LTL, LTN, MAZ, MEI,MEL, MEP, MER, MFI, MFS, MON, MOR, MSO, MTF, MTN, MTT, MTW, MWW, NAT,NES, NON, OFF, OSI, PAR, PAU, PHI, RHO, RON, RSN, RTE, RTH, RUT, SAO,SAT, SBE, SBS, SBT, SFF, SGT, SOD, STF, STI, STT, TER, THO, TON, TSC,VET, VFI, VNI, VSV, WIE, WEN, YUG, ZON structure or to mixed structurescomprising two or more of the abovementioned structures. Furthermore,titanium-containing zeolites having the ITQ-4, SSZ-24, TTM-1, UTD-1,CIT-1 or CIT-5 structure are also conceivable for use in the process ofthe present invention. Further titanium-containing zeolites which may bementioned are those of the ZSM-48 or ZSM-12 structure.

Particular preference is given to using Ti-zeolites having an MFI or MELstructure or an MFI/MEL mixed structure. Very particular preference isgiven to the specific titanium-containing zeolite catalysts which aregenerally referred to as “TS-1”, “TS-2”, “TS-3” and also Ti zeoliteshaving a framework structure isomorphous with β-zeolite.

The use of a heterogeneous catalyst comprising the titanium-containingsilicalite TS-1 is very advantageous.

In general, the reaction temperature for the preparation of thepropylene oxide in steps (α) and (γ) is in the range from 0 to 120° C.,preferably in the range from 10 to 100° C. and more preferably in therange from 20 to 90° C. The pressures which prevail range from 1 to 100bar, preferably from 1 to 40 bar, more preferably from 1 to 30 bar.Preference is given to working at pressures under which no gas phase ispresent.

The concentration of propylene and hydrogen peroxide in the feed streamis generally selected so that the molar ratio is preferably in the rangefrom 0.7 to 20, more preferably in the range from 0.8 to 5.0,particularly preferably in the range from 0.9 to 2.0 and in particularin the range from 1.0 to 1.6.

In the preparation of propylene oxide, the residence times in thereactor or reactors depend essentially on the desired conversions. Ingeneral, they are less than 5 hours, preferably less than 3 hours, morepreferably less than 1 hour and particularly preferably about half anhour.

To reduce the hydroperoxy alcohols present in the product mixture fromthe preparation of propylene oxide, it is possible to employ the methodsdescribed in DE 101 05 527.7.

For example, phosphorus(III) compounds such as phosphorus trichloride,phosphines (e.g. triphenylphosphine, tributylphosphine), phosphorousacid or its salts or sodium hyphophosphite can be used.

Reduction using sulfur(II) compounds such as hydrogen sulfide or saltsthereof, sodium polysulfides, dimethyl sulfide, tetrahydrothiophene,bis(hydroxyethyl)sulfide or sodium thiosulfate or using sulfur(IV)compounds such as sulfurous acid and its salts, sodium bisulfite orthiourea-S-oxide is also possible.

Further reducing agents are nitrites, for example, sodium nitrite orisoamyl nitrite. α-Hydroxycarbonyl compounds such as hydroxyacetone,dihydroyactone, 2-hydroxycylopentanone (glutaroin),2-hydroxycyclohexanone (adipoin), glucose and other reducing sugars arealso suitable. Ene diols such as ascorbic acid or compounds whichcontain a boron-hydrogen bond, for example, sodium borohydride or sodiumcyanoborohydride, can likewise be used.

However, the product mixtures comprising a-hydroperoxy alcohols arepreferably reduced by means of catalytic hydrogenation using hydrogen,which can be carried out in a homogeneous or heterogeneous phase. Thehydrogenation catalyst comprises at least one active metal fromtransition groups VIb, VIII, Ia and Ib of the Periodic Table of theElements, either individually or as a mixture of two or more thereof.For example, palladium (Pd), platinum (Pt), rhodium (Rh), ruthenium(Ru), iridium (Ir), osmium (Os), iron (Fe), cobalt (Co), nickel (Ni) andcopper (Cu), preferably Pd, Pt, Rh, Ru and Ir, particularly preferablyPd, can be used. These catalysts can be used either in powder form or asbodies made of active metal. Preference is given to using foils, wires,meshes, granules and crystallized powders produced from at least oneactive metal or a mixture of two or more thereof. It is also possible touse active metal oxides, for example, as suspensions of at least oneactive metal or a mixture of two or more thereof.

The present invention further provides an apparatus for carrying out acontinuous process for preparing propylene glycols comprising at leastone reactor for preparing propylene oxide, at least one reactor forreacting the propylene oxide with water to form propylene glycols, atleast one dewatering apparatus for dewatering the water-containingpropylene glycols and at least one dividing wall column having two sideofftakes for separating off propylene glycol and dipropylene glycol anda column which is thermally coupled therewith for separating off thetripropylene glycol.

In a particularly preferred embodiment of the apparatus of the presentinvention, the at least one reactor consists of an isothermal fixed-bedreactor for carrying out the step (α), an adiabatic fixed-bed reactorfor carrying out the step (γ) and a separation apparatus for carryingout the step (β).

The reactions of propylene with hydrogen peroxide in the steps (α) and(γ) are carried out in the isothermal reactor and the adiabatic reactor,respectively. Unreacted hydrogen peroxide from step (α) is separated offin step (β) in the separation apparatus and is reacted once again withpropylene in step (γ).

List of Reference Numerals for the FIGURE

1 Combined region of the inflow and offtake part of the dividing wallcolumn 2 Enrichment section of the inflow part 3 Stripping section ofthe offtake part 4 Stripping section of the inflow part 5 Enrichmentsection of the offtake part 6 Combined region of the inflow and offtakepart 7 Region of thermal coupling 8 Dividing wall P Propylene A Zone inwhich propylene oxide is prepared PO(1) Crude propylene oxide B Work-upzone for propylene oxide PO(2) Purified propylene oxide G Propyleneglycols as secondary yield C Zone in which propylene oxide is reactedwith water D Dewatering zone for the propylene glycols Z Feed point LSLow boilers PG Propylene glycol DPG Dipropylene glycol TPG Triproyleneglycol HS High boilers K Condenser V Vaporizer d Vapor f Liquid

Horizontal and diagonal or indicated diagonal lines in the columnssymbolize packing made up of random packing elements or ordered packingwhich may be present in the column.

1. A process for the continuous preparation of propylene glycols, whichcomprises the steps (i) to (iii): (i) reacting propylene with hydrogenperoxide to give a mixture comprising propylene oxide, and monopropyleneglycol, dipropylene glycol and tripropylene glycol as by-products,wherein from this mixture, a mixture comprising monopropylene glycol,dipropylene glycol and tripropylene glycol is separated via the bottomsand crude propylene oxide is separated via the top in a distillationcolumn; (ii) reacting the crude propylene oxide obtained in step (i)with water to give a mixture comprising monopropylene glycol,dipropylene glycol and tripropylene glycol; (iii) combining thepropylene glycol mixtures obtained in steps (i) and (ii) and separatingoff the respective propylene glycols by distillation, wherein in (i), anaqueous hydrogen peroxide solution is used and wherein water is removedfrom the mixture obtained in (ii) prior to combination and separation instep (iii).
 2. The process as claimed in claim 1, wherein the reactionof propylene with hydrogen peroxide in step (i) comprises at least thesteps (α) to (γ): (α) reacting propylene with hydrogen peroxide to givea mixture comprising propylene oxide and unreacted hydrogen peroxide,(β) separating the unreacted hydrogen peroxide from the mixtureresulting from step (α), (γ) reacting the hydrogen peroxide which hasbeen separated off in stage (β) with propylene.
 3. The process asclaimed in claim 1, wherein propylene glycol is obtained as by-productin step (i) by reduction of 2-hydroperoxy-1-propanol and1-hydroperoxy-2-propanol.
 4. The process as claimed in claim 1, wherein,in step (ii), propylene oxide is reacted with water at a temperature offrom 180 to 220° C. and a pressure of from 15 to 25 bar.
 5. The processas claimed in claim 1, wherein the separation in step (iii) is carriedout by distillation in a dividing wall column having two side offtakesand a column which is thermally coupled therewith, with monopropyleneglycol being obtained from the upper side offtake of the dividing wallcolumn, dipropylene glycol being obtained from the lower side offtakeand tripropylene glycol being obtained from the column which isthermally coupled therewith.
 6. The process as claimed in claim 5,wherein the distillation in the dividing wall column is carried out at apressure of from 5 to 500 mbar and a temperature of from 50 to 200° C.7. The process as claimed in claim 5, wherein the distillation in thethermally coupled column is carried out at a pressure of from 5 to 500mbar and a temperature of from 100 to 200° C.
 8. The process as claimedin claim 5, wherein the distillation in the dividing wall column iscarried out at a pressure of from 5 to 500 mbar and a temperature offrom 50 to 200° C. and wherein the distillation in the thermally coupledcolumn is carried out at a pressure of from 5 to 500 mbar and atemperature of from 100 to 200° C.
 9. A process for the continuouspreparation of propylene glycols, which comprises the steps (i) to(iii): (i) reacting propylene with hydrogen peroxide to give a mixturecomprising propylene oxide, and monopropylene glycol, dipropylene glycoland tripropylene glycol as by-products, wherein from this mixture, amixture comprising monopropylene glycol, dipropylene glycol andtripropylene glycol is separated via the bottoms and crude propyleneoxide is separated via the top in a distillation column; (ii) reactingthe crude propylene oxide obtained in step (i) with water to give amixture comprising monopropylene glycol, dipropylene glycol andtripropylene glycol; (iii) combining the propylene glycol mixturesobtained in steps (i) and (ii) and separating off the respectivepropylene glycols by distillation, wherein in (i), an aqueous hydrogenperoxide solution is used and the reaction of propylene with hydrogenperoxide comprises at least the steps (α) to (γ): (α) reacting propylenewith hydrogen peroxide to give a mixture comprising propylene oxide andunreacted hydrogen peroxide, (β) separating the unreacted hydrogenperoxide from the mixture resulting from step (α), (γ) reacting thehydrogen peroxide which has been separated off in stage (β) withpropylene, and wherein water is removed from the mixture obtained in(ii) prior to combination and separation in step (iii), which separationin (iii) is carried out by distillation in a dividing wall column havingtwo side offtakes and a column which is thermally coupled therewith,with monopropylene glycol being obtained from the upper side offtake ofthe dividing wall column, dipropylene glycol being obtained from thelower side offtake and tripropylene glycol being obtained from thecolumn which is thermally coupled therewith.
 10. The process as claimedin claim 9, wherein the distillation m the dividing wall column iscarried out at a pressure of from 5 to 500 mbar and a temperature offrom 50 to 200° C. and wherein the distillation in the thermally coupledcolumn is carried out at a pressure of from 5 to 500 mbar and atemperature of from 100 to 200° C.
 11. The process as claimed in claim9, wherein propylene glycol is obtained as by-product in step (i) byreduction of 2-hydroperoxy-1-propanol and 1-hydroperoxy-2-propanol. 12.The process as claimed in claim 9, wherein, in step (ii), propyleneoxide is reacted with water at a temperature of from 180 to 220° C. anda pressure of from 15 to 25 bar.
 13. A process for the continuouspreparation of propylene glycols, which comprises the steps (i) to(iii): (i) reacting propylene with hydrogen peroxide to give a mixturecomprising propylene oxide, and monopropylene glycol, dipropylene glycoland tripropylene glycol as by-products, wherein from this mixture, amixture comprising monopropylene glycol, dipropylene glycol andtripropylene glycol is separated via the bottoms and crude propyleneoxide is separated via the top in a distillation column; (ii) reactingthe crude propylene oxide obtained in step (i) with water at atemperature of from 180 to 220° C. and a pressure of from 15 to 25 barto give a mixture comprising monopropylene glycol, dipropylene glycoland tripropylene glycol; (iii) combining the propylene glycol mixturesobtained in steps (i) and (ii) and separating off the respectivepropylene glycols by distillation, wherein in (i), an aqueous hydrogenperoxide solution is used, propylene glycol is obtained as by-product byreduction of 2-hydroperoxy-1-propanol and 1-hydroperoxy-2-propanol, andthe reaction of propylene with hydrogen peroxide comprises at least thesteps (α) to (γ): (α) reacting propylene with hydrogen peroxide to givea mixture comprising propylene oxide and unreacted hydrogen peroxide,(β) separating the unreacted hydrogen peroxide from the mixtureresulting from step (α), (γ) reacting the hydrogen peroxide which hasbeen separated off in stage (β) with propylene, and wherein water isremoved from the mixture obtained in (ii) prior to combination andseparation in step (iii), which separation in (iii) is carried out bydistillation in a dividing wall column having two side offtakes and acolumn which is thermally coupled therewith, wherein the distillation inthe dividing wall column is carried out at a pressure of from 5 to 500mbar and a temperature of from 50 to 200° C. and wherein thedistillation in the thermally coupled column is carried out at apressure of from 5 to 500 mbar and a temperature of from 100 to 200° C.,with monopropylene glycol being obtained from the upper side offtake ofthe dividing wall column, dipropylene glycol being obtained from thelower side offtake and tripropylene glycol being obtained from thecolumn which is thermally coupled therewith.